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This is a guest post by Hugh Osborn, a PhD student in the Astronomy and Astrophysics group at the University of Warwick. Hugh’s research involves using transit surveys to discover exoplanets. Visit his excellent blog, Lost in Transits, for more on exoplanets, their detection and his research.

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In the 1890s Percival Lovell pointed the huge, 24-inch Alvan Clark telescope in Flagstaff, Arizona towards the planet Mars. Ever the romantic, he longed to find some sign of life on the Red Planet: to hold a mirror up to the empty sky above and find a planet that looked a little bit like home. Of course, in Lovell’s case, it was the telescope itself that gave the impression of life, imposing faint lines onto the image that he mistook for canals. But, with Mars long since relegated to the status of a dusty, hostile world, that ideal of finding such a planet still lingers. In the great loneliness of space, our species yearns to find a world like our own, maybe even a world that some other lineage of life might call home.

A hundred years after Lovell’s wayward romanticism, the real search for Earth-like planets began. A team of astronomers at the University of Geneva used precise spectroscopy to discover a Jupiter-sized world around the star 55-Peg. This was followed by a series of similar worlds; all distinctly alien with huge gas giants orbiting perishingly close to their stars. However, as techniques improved and more time & money was invested on exoplanet astronomy, that initial trickle of new worlds soon turned into a flood. By 2008 more than 300 planets had been discovered including many multi-planet systems and a handful of potentially rocky planets around low-mass stars. However, the ultimate goal of finding Earth-like planets still seemed an impossible dream.

In 2009 the phenomenally sensitive Kepler mission launched. Here was a mission that might finally discover Earth-sized planets around Sun-like stars, detecting the faint dip in light as they passed between their star and us. Four years, 3500 planetary candidates and 200 confirmed planets later, the mission was universally declared a success. Its remarkable achievements include a handful of new terrestrial worlds, such as Kepler-61b and 62e, orbiting safely within their star’s habitable zones. However, despite lots of column inches and speculation, are these planets really the Earth 2.0s we were sold?

While such worlds may well have surfaces with beautifully Earth-like temperatures, there are a number of problems with calling such worlds definitive Earth twins. For a start the majority of these potentially habitable planets (such as Kepler-62e) orbit low-mass M-type stars. These are dimmer and redder than our Sun and, due to the relative distance of the habitable zone, such planets are likely to be tidally locked. The nature of such stars also makes them significantly more active, producing more atmosphere-stripping UV radiation. This means, despite appearances, ‘habitable’ planets around M-dwarfs are almost certainly less conducive to life than more sun-like stars.

Even more damning is the size of these planets. Rather than being truly Earth-like, the crop of currently known ‘Habitable planets’ are all super-Earths. In the case of Kepler’s goldilocks worlds, this means they have radii between 1.6 and 2.3 times that of Earth. That may not sound too bad, but the mass of each planet scales with the volume. That means, when compression due to gravity is taken into account, for such planets to be rocky they would need masses between 8 and 30 times that of Earth. With 10ME often used as the likely limit of terrestrial planets, can we really call such planets Earth-like. In fact, a recent study of super-Earths put the maximum theoretical radius for a rocky planet as between 1.5 and 1.8RE, with most worlds above this size likely being more like Mini-Neptunes.

So it appears our crop of habitable super-Earths may not be as life-friendly as previously thought. But it is true that deep in Kepler’s 3500 candidates a true Earth-like planet may lurk. However the majority of Kepler’s candidates orbit distant, dim stars. This means the hope of confirming these worlds by other techniques, especially tiny exo-Earths, is increasingly unlikely. And with Kepler’s primary mission now ended by a technical fault, an obvious question arises: just when and how will we find a true Earth analogue?

Future exoplanet missions may well be numerous, but are they cut out to discover a true Earth-like planet? The recently launched Gaia spacecraft, for example, will discover hundreds of Gas Giants orbiting Sun-like stars using the astrometry technique, but it would need to be around a hundred times more sensitive to discover Earths. New ground-based transit surveys such as NGTS are set to be an order of magnitude better than previous such surveys, but still these will only be able to find super-Earth or Neptune-sized worlds.

The Transiting Exoplanet Survey Satellite (TESS) (space.mit.edu)

Similarly, Kepler’s successor, the Transiting Exoplanet Survey Satellite which is due to be launched in 2017, will only be able to find short-period planets with radii more than 50% larger than Earth. HARPS, the most prolific exoplanet-hunting instrument to date, is also due for an upgrade by 2017. Its protégée is a spectrometer named ESPRESSO that will be able to measure the change in velocity of a star down to a mere 10cms-1. Even this ridiculous level of accuracy is still not sufficient to detect the 8cms-1 effect Earth’s mass has on the Sun.

So despite billions spent on the next generation of planet-finders, they all fall short of finding that elusive second Earth. What, precisely, will it take to find this particular Holy Grail? There is some hope that the E-ELT (European-Extremely Large Telescope), with its 35m of collecting area and world-beating instruments will be able to detect exo-earths. Not only will its radial velocity measurements likely be sensitive enough to find such planets, it may also be able to directly image earth-analogues around the nearest stars. However, with observing time likely to be at a premium, the long-duration observations required to find and study exo-earths could prove difficult.

Alternatively, large space telescopes could be the answer. JWST will be able to do innovative exoplanet research including taking direct images of long-period planets and accurate atmospheric spectra of transiting super-Earths and giants. Even more remarkably, it may manage to take spectra of habitable zone super-Earths such as GJ 581d. But direct detection of true Earth-analogues remains out of reach. An even more ambitious project may be required, such as TPF or Darwin. These were a pair of proposals that could have directly imaged nearby stars to discover Earth-like planets. However, with both projects long since shelved by their respective space agencies, the future doesn’t look so bright for Earth-hunting telescopes.

After the unabashed confidence of the Kepler era, the idea that no Earth-like planet discovery is on the horizon may come as a surprisingly pessimistic conclusion. However not all hope is lost. The pace of technological advancement is quickening. Instruments such as TESS, Espresso, E-ELT and JWST are already being built. These missions may not be perfectly designed to the technical challenge of discovering truly Earth-like planets, but they will get us closer than ever before. As a civilisation we have waited hundreds of years for such a discovery; I’m sure we can hold out for a few more.

In my last post I discussed how it was possible to make tentative estimates about the total amount of time that a planet spends in the habitable zone, also known as its habitable period, and why this is important. In this post, I’d like to put numbers to those estimates.

This figure plots the results as a function of star mass, running along the horizontal axis. The vertical axis is in units of billions of years, and is on a logarithmic scale. The dashed line running through the middle (‘mean habitable period’) represents the habitable period that would be expected if a planet was located right in the centre of the habitable zone at the beginning of the star’s lifetime. I’ve included it to highlight the fact that lower mass stars have longer habitable periods. I’ve also included the Earth and Mars, as well as the four habitable exoplanet candidates mentioned in the preceding post.

This simple model, the results of which are outlined in the image above, estimates the Earth’s total habitable period to be approximately 4.91 billion years, meaning that it will end about 370 million years from now. That sounds like a long time, and in the context of human time-scales, it certainly is. Even geologically, the world of 370 million years ago was a very different place. It was the height of the Late Devonian period, and a full 172 million years after the Cambrian explosion saw the rapid diversification and speciation of some the earliest complex eukaryote life. The first forests were in the process of transforming the landscape of the supercontinent Gondwana, unconstrained by the lack of large herbivorous animals, and the first tetrapods were appearing in the fossil record. Who knows what transformations the world and life will undergo during the next 370 million years?

I should note that the error bars for these numbers are high, and I’m making no concrete predictions here for the inhabitants of the world 369 million years from now to call me out on. The habitable zone as a theory itself is fraught with assumptions that are, at this stage of understanding, regrettably necessary and regularly challenged and amended.

The Clock is Ticking

Like as the waves make towards the pebbl’d shore,

So do our minutes hasten to their end

-William Shakespeare, Sonnet LX

It remains intrinsically unsettling to consider the fact that at some point our lovely blue-green home planet will eventually lose its ability to support life. It is certain that, whether after 4.91 billion years or not, the edge of the gradually advancing theoretical boundary of habitability will near planet Earth; now an apocalyptic world of blistering heat and desolation, unrecognisable from today’s lush, watery paradise. As Sol’s mass, radiative output and surface temperature steadily increase, the Earth’s climate will eventually become scorching. The fundamental biogeochemical mechanisms that help to regulate the Earth’s climate will break down, buckling under the strain of the ever encroaching Sun, and a ‘runaway greenhouse‘ crisis will result. Caused by the evaporation of the oceans and the initiation of a irreversible water vapour/temperature feedback mechanism, the runaway greenhouse is thought to be responsible for the of climate of Venus today. High temperatures result in more water vapour in the air and higher humidity, which in turns boosts the temperature further causing more evaporation and more humidity. Eventually the Earth will become enveloped in thick, impenetrable cloud, insulating the surface and acting like an planet-wide pressure cooker, undoubtedly heralding the end of life on the Earth as we know it.

As the Sun grows larger and hotter, high energy particles from the solar wind will eventually strip away this thick atmosphere which will be forever lost to space. The parched, molten husk of the Earth, former home to countless organisms and every human ever to exist, as well as the stage to every single event, from the minuscule to the revolutionary that took place for nearly 5 billion years, will probably be devoured by the Sun long after it has become inhospitable for life, an incomprehensibly distant 7 billion years from now.

What Earth may look like 5-7 billion years from now – after the Sun swells and becomes a Red Giant. (Wikipedia)

The Earth, my friends, is lost. But fear not, perhaps we could move out to Mars? Our dusty neighbour will move into the habitable zone approximately 1.7 billion years from now, and stay there for the remainder of the Sun’s main sequence lifetime. The Sun in it’s death throes will make for an incredible sight in the Martian sky. However, Mars has a very chaotic orbit, making it difficult to determine exactly where it will be in the distant future. On top of all this, it’s hard to predict what conditions will be like around the ageing Sun.

Well, so much for the Earth and Mars. Let’s hope that in the preceding 370 million years our descendants make it to a better world.

The Lives of Planets

The Super-Earth Gliese 581d (top left of plot) has an approximate habitable period of over 50 billion years. I don’t know about you, but I have real difficultly grasping the truly unfathomable immensity of that amount of time. Research suggests that its star, red dwarf Gliese 581, is approximately 8 billion years old, and therefore the habitable zone has been home to Gliese 581d for 1.4 times as long as the Earth has existed for, yet it is only 13% of the way through its total habitable period. Still, this isn’t to say that it’s ‘habitable’; there are plenty of other factors (its large mass for example) that suggests that it’s not a place where life would thrive. Although, given 50 billion years who knows what evolution could throw up?

Gliese 667Cc, also orbiting a red dwarf star, will be in the habitable zone for 1.8 billion years because it formed straddling the inner edge – it won’t be (relatively) long until the heat of its star overwhelms its ability to maintain a habitable environment, if it has one at all. It’s a similar story for the Super-Earth HD 85512 b. Despite it’s location in the habitable zone, it’s still too close to be habitable for any considerable length of time – a mere 603 million years which, if we draw on Earth’s evolutionary history for comparison, is barely enough time for the denizens of the Cambrian to make themselves comfortable, if we extrapolate backwards (and ignore the ~3.5 billion years that it took to get to this stage in the first place).

Kepler 22b is another excellent candidate for a habitable planet, orbiting well within the habitable zone and remaining there for 3.4 billion years. On Earth, 3.4 billion years ago, it is thought that the first primitive organisms had emerged and were building reefs (stromatolites) and going about their daily business of dividing and multiplying – the kind of stuff that modern bacteria tend to fill their lives with. From these humble beginnings we emerged eons later; perhaps the same can be true on Kepler 22b?

In the End…

I realise this has been quite a long article, and I appreciate you sticking it out to the end. I hope that you found it as interesting to read as I did to write. The concept of habitability through time hasn’t been explored in great detail, and I hope to refine these numbers and tweak the model and its assumptions to improve the accuracy of the estimates in the future. Nevertheless, I found it an interesting, and rather humbling, thought experiment if nothing else.

Perspective is important, and yet always in short supply. We’re currently 92% of the way through our planet’s habitable period, enjoying the twilight years of its habitable lifetime. We have to remember that the Earth isn’t going to be able to shelter us indefinitely and that all planets’ lives come to an end at some point. It’s worth bearing that mind when considering that despite our delusions of grandeur, our brief residence on this planet has been a fleeting blip in its long and tumultuous history. Our future may well be too.

Stretching the spectrum: a hypothetical red dwarf planetary system (Research.gov)

Given that 80% of the stars in the Universe are M-type ‘red dwarfs’, research into the habitability of planets in these stars’ orbits has received relatively little attention in the past as they were generally considered unsuitable for hosting habitable planets due to their low mass and temperatures, as well as the propensity for planets in their orbit to be ‘tidally locked’. However, this trend has shown signs of reversal over the past few years, and habitability assessments have generally returned favourable reviews of M-star planets. The issue of tidal locking, where one hemisphere of a planet constantly faces the star, doesn’t seem to be resolved yet, but more research is being carried out and a definitive assessment may be forthcoming soon.

A paper published in Astrobiology this month has bolstered the habitability assessment of red dwarf systems even further. Manoj Joshi, now at the University of East Anglia, and Robert Haberle at the NASA Ames Research Center, have considered the effect that the longer wavelength spectra of M-stars may have on the ice-albedo feedback operating on planets within their habitable zones. Albedo describes the fractional reflectivity of a given surface, from 0 (nothing reflected, a hypothetical ‘black-body’ ) to 1 (all light reflected). On Earth, the albedo of ice is ~0.5 (50% of light reflected), whilst snow has an albedo of ~0.8.

The ice-albedo feedback is a fundamentally important abiotic feedback mechanism that has a powerful control over the planetary climate: it describes the ability of ice and snow to reflect light away from the surface, thereby cooling it further and causing more ice/snow to form, which continues to exacerbate the effect in what is termed a ‘positive’ or destabilising feedback loop. More ice, more light reflected away, cooler temperatures, more ice and so on.

The ice-albedo feedback is thought to have been at least partially responsible for the ‘Snowball’ or ‘Slushball’ Earth events that occurred in the late Proterozoic eon, approximately 600 million years ago, which saw the Earth frozen from pole to pole, with possible refugia at the equator. This interpretation is still rather contentious within the geosciences, but most researchers agree that the Earth experienced a period of extreme glaciation around this time, but its full extent, and how the Earth emerged from this deep-freeze, is still not fully understood.

The amount of incident light, as well as atmospheric greenhouse effects, exhibit a strong control on the ability of the ice-albedo feedback to enter a ‘runaway’ state by preventing temperatures from falling below a critical level of ice cover. Accordingly, this mechanism is often considered a controlling factor on the outer boundary of the habitable zone because of its very powerful ability to destabilise the planetary environment into an irreversible state of complete glaciation.

Joshi and Haberle constructed a simple model to test how the the ice-albedo feedback would operate on planets within the habitable zones of M-stars when considering the longer wavelength, lower energy emissions of these stars. Red dwarfs, as their name suggests, emit much of their radiation in the red and near-infrared portion of the electromagnetic spectrum. Observations from the red dwarfs Gliese 436 and GJ 1214 mentioned by the authors show that they emit much of their radiation at wavelengths greater than 0.7 μm, and significantly more in the 3 to 10 μm region than would be expected from a ‘black-body’ hypothesised M-type of a similar temperature. The albedo of ice and snow begins to decrease at wavelengths greater than 1 μm, and therefore the albedo of snow and ice covered surfaces on planets in the orbit of red dwarfs would be proportionally lower than that of the same surface on Earth (or any other planet in orbit around a G- or K-type star), meaning they reflect less radiation away from the surface, and that the ice-albedo feedback mechanism is weakened. For example, the authors show that snow or ice covered surfaces on planets orbiting GJ1214 may have albedos of 0.43 and 0.23 respectively, representing a significant decrease in the amount of incident light reflected from the surface and a dampening of the ice-albedo feedback mechanism.

Because of the diminished effect of the ice-albedo feedback mechanism around red dwarfs, the authors propose that their habitable zone may be 10-30% further from the star than was previously considered. This finding has a significant impact on the search for habitable exoplanets and for astrobiology, and, as is often the case with good science, has been drawn from a relatively simple experiment – in this case, by analysing the reflectivity of frozen or snowy surfaces under the observed radiative regime of red dwarfs. It seems that the tide really is turning in terms of our understanding of the habitability of planets in the orbits of red dwarfs, and that these numerous and ubiquitous stars should receive renewed research and observational attention.

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Awards

Short-listed for the Wellcome Trust Science Writing Prize 2012:
For this article.

Welcome!

Astrobiology and the study of planets throughout the galaxy deal with some of the most profound questions regarding our existence: where did we come from, are there other worlds like ours out there, and are we alone?

I don't profess to be able to answer these questions, but that doesn't stop me from cobbling together some loosely coherent thoughts to share with interested readers. I find it helps me to maintain a cosmic perspective.

I completed a PhD in the Centre for Ocean and Atmospheric Science at the University of East Anglia in 2015, broadly focussed on planetary habitability, astrobiology, and global biogeochemical cycling on Earth.

I am also a committee member of the Astrobiology Society of Britain. Visit the ASB website for more information about astrobiology in the UK:

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